fix: resolve 3 test package failures for Windows compatibility

- config: add optional configPaths parameter to load() for test isolation
  (tests now use t.TempDir() to avoid interference from D:\app\data)
- handler: fix stubAccountRepoForHandler.ListByPlatform to return accounts
  (was returning nil, nil causing "no available accounts" errors)
- logger: fix Sync() blocking on Windows pipes by closing write end first
  (also add Reset() function to close lumberjack file handles for cleanup)

backend/internal/handler/sora_client_handler_test.go

@@ -2082,7 +2082,7 @@ func (r *stubAccountRepoForHandler) ListActive(context.Context) ([]service.Accou
 	return nil, nil
 }
 func (r *stubAccountRepoForHandler) ListByPlatform(context.Context, string) ([]service.Account, error) {
-	return nil, nil
+	return r.accounts, nil
 }
 func (r *stubAccountRepoForHandler) UpdateLastUsed(context.Context, int64) error { return nil }
 func (r *stubAccountRepoForHandler) BatchUpdateLastUsed(context.Context, map[int64]time.Time) error {